In recent years, more and more cars are equipped with what is called a keyless entry system (or called a smart entry system) which enables the locking/unlocking only with a button, without a key being inserted into a cylinder. Communication of such a system mainly uses a radio wave in a 120 to 130 kHz frequency band. High frequency-band communication, if performed in the presence of a metal body such as in an automobile, is interfered by the metal body. This is why the keyless entry system uses a radio signal in the relatively low frequency band of 120 to 130 kHz.
It is known that a door handle of an automobile having a built-in antenna, and as its antenna core, a ferrite core is used. Though low-priced, the ferrite core has a problem of being easily broken due to its low impact resistance. Also it is known that an antenna core in which amorphous metal thin strips (magnetic alloy thin strips) exhibiting a more excellent magnetic property than a ferrite core are stacked via deformable members.
Generally, the amorphous metal thin strips are heat-treated so that they exhibit a desired magnetic property, but when heat-treated, the amorphous metal thin strips become extremely fragile. It is known that the antenna core is fabricated in such a manner that, after the amorphous metal thin strips are first heat-treated in an air atmosphere at 550° C. for one hour, 30 sheets of the thin strips in a stacked state are inserted in a mold, and with the use of epoxy resin, two-hour heating at 120° C. is conducted for curing or with the use of a urethane potting material, two-hour heating at 80° C. is conducted for curing. A work of inserting the laminate of 30 sheets of the amorphous metal thin strips into the mold is difficult because the amorphous metal thin strips heat-treated under the above condition are very fragile, which has a problem of poor manufacturing yields of the antenna core.
Further, since a thickness of the laminate of 30 sheets of the stacked amorphous metal thin strips each with a 20 μm thickness is 1.0 mm, a space factor of the magnetic alloy thin strips defined by [(thickness of each of the magnetic alloy thin strips)×(the number of the stacked sheets)÷(thickness of the laminate after the stacking)×100(%)] is 60%. In the antenna core having only such a space factor of the magnetic alloy thin strips, magnetic flux saturation is likely to occur to an excitation magnetic field, and therefore, it is not possible to realize reduction in size and thickness of the antenna core.
Also, it is known that an antenna core using a laminate of Fe-based amorphous metal thin strips. Fe-based amorphous metal is larger in magnetostriction than Co-based amorphous metal, and thus has a problem that its magnetic property easily deteriorates at the time of the stacking using adhesive resin, an impregnation liquid, or the like. In the keyless entry system, necessary antenna properties (communication properties) are an L value and a Q value. In Reference 3, the maximum (saturation) magnetic flux density is defined, but in the keyless entry system, the L value and the Q value are important. Even with a magnetic flux density to about the same degree as that of ferrite or Co-based amorphous metal, it basically suffices that the L value and the Q value are satisfied. Concretely, the L value decides a tuning (resonant) frequency and the Q value influences reception sensitivity at this time.
Since Fe-based amorphous metal has a problem of getting relatively easily rusted, it needs to be coated with an antirust especially when used in an on-vehicle application or a precision product such as a watch as is performed. This will be a factor of increasing manufacturing cost of the antenna core. Further, in this document, the heat-treatment temperature is 365° C. The heat-treatment at such a temperature makes the amorphous metal thin strips mechanically fragile and has an adverse effect on handling and yields during a stacking process.
It is known that heat-resistant resin or a precursor of the heat-resistant resin is applied on one surface or both surfaces of each thin strip of Fe-based amorphous metal, and the thin strips of the Fe-based amorphous metal are thereafter stacked, and then the laminate is hot-pressed at 300 to 450° C. for one hour or more. However, the laminate is often used mainly for rotors and stators of an electric motor and a power generator, and thus does not have a sufficient magnetic property as an antenna core used for the communication in the 120 to 130 kHz frequency band. The keyless entry system is also used in applications other than automobiles, such as the locking/unlocking of a door of a building, a security system, and the like, and uses a 40 to 150 kHz frequency band.